1. Field of the Invention
The present invention relates to a photomask used in a photolithography process which is one of the manufacturing processes of a semiconductor device.
2. Description of the Background Art
A lithography technique is usually used for forming a circuit pattern to be provided on a semiconductor device. This technique fundamentally consists of a coating step for coating a substrate such as wafer with photoresist to form a photoresist layer, an exposure step for impinging light (for example, light in an ultraviolet region) from a suitable light source upon a photomask having a predetermined pattern to transfer the pattern to the photoresist layer and a development step for developing the photoresist layer to obtain a photoresist layer having the predetermined pattern.
FIG. 1 is a perspective view of a conventional photomask 70 used in the exposure step. In FIG. 1, numeral 71 represents a glass substrate, on which a shield layer 72 of chromium is formed. The shield layer 72 is provided with two adjacent rectagular light-transmission patterns 73 and 74 formed by rectangular openings in parallel. In addition, the term "rectangular" means a shape including a square and a oblong.
FIG. 2 is a schematic structural view of an exposure device. As shown in FIG. 2, the exposure device is provided with a light source 81 for emitting light in an ultraviolet region downwardly. The light from the light source 81 is directed through a lens 82 to the photomask 70. Some of the incident light passes through the rectangular light-transmission patterns 73 and 74, further to be introduced through a lens 83 onto a photoresist surface 84. On the other hand, the light impinging upon the shield layer 72 is shielded by the shield layer 72. Thus, exposure patterns corresponding to the rectangular light-transmission patterns 73 and 74 are transferred to the photoresist surface 84.
As in FIG. 1, the rectangular light-transmission patterns 73 and 74 are adjacent to each other; More particularly, one rectangular light-transmission pattern 74 (or 73) is disposed on the normal lines N.sub.3 (or N.sub.4) of the other rectangular light-transmission pattern 73 (or 74). When the exposure process is carried out with the photomask 70, diffraction light from the rectangular light-transmission pattern 73 (or 74) exerts adverse effect on the form of the transferred pattern which is formed by focusing diffraction light from the rectangular light-transmission pattern 74 (or 73) on the photoresist surface 84; in brief, the patterns transferred on the photoresist surface 84 are deformed by the diffraction light. The closer the rectangular light-transmission patterns 73 and 74 are in proximity to each other, the larger the deformation grows. The deformation causes the deterioration of transfer accuracy of the pattern to the photoresist surface 84. In particular, in these days of advancing high integration, the rectangular light-transmission patterns 73 and 74 are disposed in further closer proximity to each other. Accordingly, the influence of the diffraction light is a serious problem.